In large format printing, e.g. poster printing, billboard printing, sign printing, the weatherability of the print is very important. In that area silk-screen printing is still a dominant printing method. This method has however many drawbacks: first of all it is rather time consuming since for every color a dedicated screen has to be made and printed, the method is basically analogue and not well compatible with digital input files.
More and more images to be printed are available in digital form, so that also in the printing of large formats, digital addressable printing techniques become indispensable.
A well known digital addressable printing technique that is useful for large format printing is ink-jet printing, both with water based inks and with solvent based inks. An example of an ink-jet printer for large format printing can be found in, e.g. U.S. Pat. No. 5,488,397, wherein a printer is disclosed having two or more parallel ink-cartridges shuttling over the width of the substrate to be printed while the substrate moves in a direction basically perpendicular to the direction of movement of the shuttling ink-cartridges.
In WO-A-96/01489 an ink-jet printer for large format printing is disclosed wherein a single ink-cartridge shuttles over the substrate to be printed.
In U.S. Pat. No. 4,864,328 an in-jet printer is disclosed, wherein only one printing engine (ink-jet head) having a multiple array of nozzles is moved as a shuttle over the paper.
In EP-A-526 205 again an ink-jet printer is disclosed, wherein only one printing engine (ink-jet head) having a multiple array of nozzles is moved as a shuttle over the paper.
A commercial ink-jet printer IDANIT 162Ad (trade name) available from Idanit Technologies, Israel, uses multiple ink-jet printheads mounted in a staggered position over the width of the substrate to be printed. In this device the printing substrate has to pass several times under the array of staggered ink-jet printheads while between each pass the printheads are slightly moved with respect to the drum in a direction parallel to the width of the substrate. This multi-pass printing enhances the resolution that can be printed, while in the printhead itself the nozzle can be positioned fairly far apart. The same concept (but with much less printheads) has also be commercially implemented in printers such as the LASERMASTER DESIGNWINDER, IRIS REALIST, STORK TEXTILE PROOFER, POLAROID DRYJET (trade names), . . . and is e.g. further described in WO-A-96/34762.
Although ink-jet printing provides the possibility for printing large formats in short time, the resulting printing quality is not always up to the demands, the stability of the image in, e.g. billboards where the image has to be weatherproof leaves still room for improvement.
In U.S. Pat. No. 5,138,366 a thermal printer using at least two thermal printing heads is described for printing on large substrates. The maximum format for a commercially available large format printer using thermal technology, however, is 36 inch, as provided by the Matan Sprinter, Israel.
In U.S. Pat. No. 5,237,347 an electrophotographic printer is disclosed wherein a single photoconductor is exposed to the light of several exposure units, so a large latent image can be written on the photoconductor and after development be transferred to a final substrate. The printer having the largest printing width for printing full color images based on electrophotographic techniques, is e.g. the Xeikon DCP50, having a printing width of 50 cm. In electrostatic technology full color printing machines having a printing with of 54 inch are available, said devices being fed with liquid electrophotographic developer.
In WO-A-96/18506 a shuttling printer using more than one Direct Electrostatic Printing (DEP) engine is disclosed wherein these engines are placed one after the other for printing multi-color swaths.
In DEP (Direct Electrostatic Printing) toner particles are deposited directly in an image-wise way on a receiving substrate, the latter not bearing any image-wise latent electrostatic image.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible, or from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image (photoconductor and charging/exposure cycle).
A DEP device is disclosed in e.g. U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer; PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and PA1 at least one row of apertures. PA1 a printing width (PW) for printing a toner image on a substrate, the substrate having a width (WS) and a length (LS), comprising PA1 a charged toner conveyer, CTC, with a length, L.sub.CTC, parallel to said printing width, carrying charged toner particles on its surface and coupled to a voltage source so as to create a flow of charged toner particles from said surface towards said substrate, PA1 a printhead structure with an array of printing apertures and control electrodes associated therewith, said printhead structure being positioned between said CTC and said substrate and said control electrodes being coupled to a second voltage source arranged so as to image-wise modulate said flow of charged toner particles, wherein PA1 i) a single central conditioning unit for controlling and monitoring the condition of the developer is provided, and PA1 ii) said central conditioning unit is equipped with means for circulating said developer to all of said n toner applicators and back to said central unit.
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected electric potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a support for a toner receiving substrate projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving substrate, interposed in the modulated particle stream. The receiving substrate is transported in a direction perpendicular to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrodes may face the receiving substrate. A DC-field is applied between the printhead structure and a single back electrode on the receiving substrate. This propulsion field is responsible for the attraction of toner to the receiving substrate that is placed between the printhead structure and the back electrode.
In EP-A-849 087 a single pass large format printer is disclosed, having at least two printing engines (DEP engines or electrophotographic engines) which are staggered with respect to the printing direction so that a large format image can be printed which is larger in size than the printing width of one of said printing engines.
In EP-A-849-645 a large format printer is disclosed having a page wide DEP-printhead structure combined with multiple smaller sized toner applicator modules, and in EP-A-849 640 a large format printer is disclosed having a page wide photoconductor combined with multiple smaller sized toner delivery means.
In the art of printing large formats, however, slight density fluctuations between neighboring image swaths easily lead to overall image deterioration. This phenomenon can be seen in shuttle printers in which neighboring printing swaths do, although they receive the same image input, not always print at the same density. When this phenomenon appears, banding is seen in the final image. Also in page wide printers, the printout from neighboring printing units does not always have exactly the same density although all printing units are activated by the same digital image input. This leads again to the problem of uneven density and banding in the final image.
Thus there is still a need for further improved large format printing devices making it possible to print at elevated speed with no or very low banding.